This PDF file contains the front matter associated with SPIE Proceedings Volume 9923, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

While photovoltaic cells are highly promising man-made devices for direct solar energy utilization, a number of fundamental questions about how the organic bulk heterojunction cell enables efficient long-lived and long-range charge separation remain unanswered. These questions were address by employing an advanced suite of EPR spectroscopy in combination with DFT calculations to study mechanism of charge separation at the polymer-fullerene interfaces of photo-active BHJ. Observed charge delocalization in BHJ upon photoinduced ET is analogous to that in organic donor-acceptor material. This is an efficient mechanism of charge stabilization in photosynthetic assemblies. Time-resolved EPR spectra show a strong polarization pattern for all polymer-fullerene blends under study, which is caused by non-Boltzmann population of the electron spin energy levels in the radical pairs. The first observation of this phenomenon was reported in natural and artificial photosynthetic assemblies, and comparison with these systems allows us to better understand charge separation processes in OPVs. The spectral analysis presented here, in combination with DFT calculations, shows that CS processes in OPV materials are similar to that in organic photosynthetic systems. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract DE-AC02-06CH11357 at Argonne National Laboratory.

The standard view of singlet exciton fission in organic semiconductor is that one photon creates a singlet exciton which subsequently decays into a correlated triplet pair state (TT) multiexciton states. The triplet pair state then splits to form two free triplets. Although the theoretical description of (TT) is well developed since 1970, it has so far proved difficult to determine the role and nature of the (TT) state in solid films from experiment directly. Here, using a combination of highly sensitive broadband transient absorption and photoluminescence spectroscopies on a range of polyacene films, we demonstrate that the (TT) multiexciton states is bound and energetically stabilised with respect to free triplets in even the most efficient singlet fission materials, such as TIPS-pentacene and pentacene. The (TT) multiexciton state is emissive, and we find that charge-transfer from one (TT) state to the neighboring electron acceptors has a yield of >100%, i.e. more than one charge is transferred per charge-transfer event. Our findings suggest that the formation of spin-correlated (TT) states emits as one particle and generates 2 charges in organic solar cells and thus open a range of fascinating questions regarding the potential to use entanglement to enhance organic photovoltaic efficiency and the application of organic materials in quantum information

Organic solar cells rely on the conversion of a Frenkel exciton into free charges via a charge transfer state formed on a molecular donor-acceptor pair. These charge transfer states are strongly bound by Coulomb interactions, and yet efficiently converted into charge-separated states. A microscopic understanding of this process, though crucial to the functionality of any solar cell, has not yet been achieved. Here we show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority-carrier trapping across a donor-acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a tradeoff between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices.

We study the role of electron-vibration coupling, electronic polarization, molecular packing, system size and electron delocalization on the nature of the charge-transfer states in model donor-acceptor systems. The morphologies we consider range from a bilayer with flat interface to a bilayer with rough interface and bulk heterojunctions with coarse and fine intercalated domains of donor and acceptor molecules. The implications of the charge-transfer states, active material morphology, density of states and charge carrier concentration on non-geminate recombination kinetics is investigated by means of a three-dimensional reaction-diffusion lattice model with the charge carrier hopping rate described by the Miller-Abrahams formalism.

Organic semiconductors have unique optical, mechanical and electronic properties that can be combined with customized chemical functionality. In the crystalline form, determinant features for electronic applications such as molecular purity, the charge mobility or the exciton diffusion length, reveal a superior performance when compared with materials in a more disordered form. Combining crystals of two different conjugated materials as even enable a new 2D electronic system. However, the use of organic single crystals in devices is still limited to a few applications, such as field-effect transistors. In 2013, we presented the first system composed of single-crystal charge transfer interfaces presenting photoconductivity behaviour. The system composed of rubrene and TCNQ has a responsivity reaching 1 A/W, corresponding to an external quantum efficiency of nearly 100%. A similar approach, with a hybrid structure of a PCBM film and rubrene single crystal also presents high responsivity and the possibility to extract excitons generated in acceptor materials. This strategy led to an extended action towards the near IR. By adequate material design and structural organisation of perylediimides, we demonstrate that is possible to improve exciton diffusion efficiency. More recently, we have successfully used the concept of charge transfer interfaces in phototransistors. These results open the possibility of using organic single-crystal interfaces in photonic applications.

In organic electronics, the interactions at interfaces between different organic and inorganic layers play a decisive role for device functionality and performance. Therefore, more detailed, quantitative studies of charge transfer (CT) at such interfaces are needed to improve the understanding of the underlying mechanisms. In this study we show that in-situ infrared spectroscopy can be used to investigate CT effects at organic/organic as well as inorganic/organic interfaces quantitatively. For different combinations of commonly used organic semiconductors such as 4,4´-bis(N-carbazolyl)-1,1´-biphenyl (CBP) or fluorinated zinc phthalocyanine (F4ZnPc) and inorganic contact materials such as molybdenum oxide (MoO3) or indium tin oxide (ITO) the CT at the interface was investigated using in-situ IR spectroscopy. The measurements were carried out under UHV conditions during film growth what enables a careful study of the influence of different parameters such as substrate temperature and layer thickness in a controlled way even on a nanometer scale. When the organic molecules are deposited onto the underlying layer charged and non-charged species form which can be identified and quantitatively analyzed in the IR spectra. It was also found that the deposition sequence can strongly influence the interface properties what might have strong implications on the layer stack design. For example, when MoO3 is deposited onto CBP, the CBP layer is strongly doped, due to diffusion of the deposited transition metal oxide clusters into the organic layer. Financial support by BMBF (project INTERPHASE) is gratefully acknowledged.

Stark spectroscopy has been pioneered many decades ago and is a unique tool to extract information on molecular constants such as changes of dipole moments or polarizabilities upon excitation. Here we introduce a new twist, i.e. THz Stark spectroscopy. In THz fields the electric field vector oscillates on time scales of picoseconds and thus much faster than in conventional Stark spectroscopy. It therefore may allow to distinguish between different electric field contributions by analyzing the dynamics of the THz response. We first demonstrate that conventional THz sources can be boosted by combination with field enhancement structures, reaching field strength of GV/m driving several different materials into the nonlinear response regime. Then we discuss THz fields influencing charge transfer in different molecules.

Hybrid metal halide perovskites are attractive components for many optoelectronic applications due to a combination of their superior charge transport properties and relative ease of fabrication. A complete understanding of the nature of charge transport in these materials is therefore essential for current and future device development. We have evaluated two systems – the standard perovskite methylammonium lead triiodide (CH3NH3PbI3) and a series of mixed-iodide/bromide formamidinium lead perovskites – in an effort to determine what effect structural and chemical composition have on optoelectronic properties including mobility, charge-carrier recombination dynamics, and charge-carrier diffusion length. The photoconductivity in thin films of CH3NH3PbI3was investigated from 8 K to 370 K across three structural phases [1]. While the monomolecular charge-carrier recombination rate was found to increase with rising temperature indicating a mechanism dominated by ionized impurity mediated recombination, the bimolecular rate constant decreased with rising temperature as charge-carrier mobility declined. The Auger rate constant was highly phase specific, suggesting a strong dependence on electronic band structure. For the mixed-halide formamidinuim lead bromide-iodide perovskites, HC(NH2)2Pb(BryI1–y)3, bimolecular and Auger charge-carrier recombination rate constants strongly correlated with bromide content, which indicated a link with electronic structure [2]. Although HC(NH2)2PbBr3 and HC(NH2)2PbI3 exhibited high charge-carrier mobilities and diffusion lengths exceeding 1 μm, mobilities for mixed Br/I perovskites were all lower as a result of crystalline phase disorder.

In this work we investigate the mechanisms responsible for the energy level alignment at inorganic and organic semiconductors interfaces with photoelectron spectroscopy. We focus on the different contributions that lead to a substantial work function increase (up to 2.5 eV) when depositing thin layers of organic acceptor molecules [1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile (HATCN) and 2,2'-(perfluoronaphthalene-2,6- diylidene)dimalononitrile (F6-TCNNQ)] on two different inorganic semiconductors (ZnO and GaN). We discuss models to describe this work function increase, which take into account the role of bulk donor concentration of the inorganic semiconductors, their surface state density, and the distance between the inorganic semiconductor and the adsorbed molecules, and we emphasize the importance of surface states on the inorganic semiconductor. The absence of any adsorption-induced core level features for F6-TCNNQ indicates fractional charge transfer, in contrast to HATCN, where additional core level features indicate integer charge transfer. Finally, we demonstrate the utility of this interlayer approach by changing the energy level alignment between the showcase hybrid system ZnO/Sp6

ZnO is attracting significant interest as a candidate for hybrid photovoltaic and light-emitting devices. We studied electronic coupling at interfaces of ZnO with conjugated organic molecules like ladder-type oligo(phenylenes) (LOP) and NTCDA whose fundamental optical excitations are resonant to the ZnO band gap as well as with polymers employing a combination of time-resolved techniques as well as in situ differential reflectance and photoemission spectroscopy. Our studies provide evidence for the formation of hybrid charge transfer excitations (HCTE) across (Zn,Mg)O/organic interfaces. We show that by interfacial design the properties of these HCTE can be tuned and by that the charge separation process. The impact of the HCTE on photovoltaic parameters like the open circuit voltage and short circuit current is exemplarily demonstrated in (Zn,Mg)O/P3HT diodes. Furthermore, we show that by proper alignment of the frontier molecular orbitals with the semiconductor valence and conduction band edges, exciton dissociation at the interface can be switched off while exciton transfer efficiencies of up to 80 % are maintained. Thus, efficient conversion of ZnO excitons into highly emissive excitons of the organic (LOP) layer is achieved which is essential for the realization of hybrid light-emitting diodes.

The ability to downconvert (1 photon to 2 photons) and upconvert (2 photons to 1 photon) energy can have applications in many fields, including solar energy. Singlet fission provides a way to convert one photon into a pair of triplet excitons. It occurs efficiently in organic semiconductors, but the question remains how to extract the triplet excitons in a useful form. In this talk, we will describe efforts to transform triplet excitons into other forms through energy transfer into inorganic semiconductors like silicon. Heterogeneous solid-liquid approaches to use spin-orbit coupling to enhance the triplet excitons’ oscillator strength so they can emit photons will also be described. The solid-solid and solid-liquid interface appears to be critical for these schemes to succeed. Upconversion occurs via the reverse process, where a pair of triplet excitons fuse into a high-energy singlet state. A new approach to triplet state sensitization involves absorption of low energy photons by the semiconductor nanocrystals followed by energy transfer to the molecular triplet states. These states can then undergo triplet-triplet annihilation to create high energy singlet states that emit upconverted light in the visible and ultraviolet regions. By using conjugated organic ligands to form an energy cascade, the upconversion can be enhanced by up to three orders of magnitude. The mechanism of the nanocrystal-to-triplet energy transfer is investigated using time-resolved spectroscopy. Again, the role of organic ligand-inorganic surface interactions is important for determining the ultimate efficiency.

It is crucial for the efficiency of photocatalytic reactions how to separate the photo-excited electrons and holes and how to utilize them at interfaces. There are two main difficulties to make these possible; variations of defects and co-catalysts. Most of the metal oxide photocatalysts have shallow and deep trap states, whose structure is always controversial. It is hard to tell which state helps reaction or not. Various co-catalysts have been applied, but also it is difficult to tell the real effect; charge separation or the surface passivation. Here, we will show the method to distinguish the defect type from the electron dynamics by using the transient grating (TG) method, which has a high sensitivity at the interface. We prepared a film sample by heating a TiO2 paste on a glass substrate. The film was contacted with a reactant solution sandwiched by another glass and a spacer. The TG method is one of the time-resolved techniques, which measures the refractive index change at the interface after shining a pulse excitation light. We could distinguish three different routes for photo-excited electrons; bulk trap (<100 ns), surface trap (1-5 us), Ti trap (0.5 – 10 s). Only the surface trap showed reactivity with reactants on the solution side. Ti trap had a longer lifetime, which was only observed when the photoexcited holes were scavenged. This trap seems to affect the cycle of the photocatalyst. This method offers simultaneous measurements of different trap states, and gives an insight of which defects have an actual reactivity.

Small organic molecules of the push-pull architecture are rapidly gaining their status in the organic electronics applications. In densely packed molecular films, both intra- and intermolecular interactions play an essential role for the device performance. Here we study two different molecules, a highly symmetric star-shaped one and its newly synthesized single arm analogue, for their photophysical properties. Both chromophores were dissolved in a solid matrix at different concentrations to vary their separation and therefore intermolecular coupling. We show that in both molecules the population relaxation accelerates by more than a factor of 10 at shorter intermolecular distances due to self-quenching thereby reducing the exciton survival time. The transient anisotropy dynamics are also quite similar, with their substantial acceleration at shorter interchromophore distances due to exciton diffusion caused by the Förster-like resonance energy transfer. However, the anisotropy values are noticeably lower for the star-shaped molecule because of intramolecular mixing of different polarization states. Finally, a model is presented that accounts for the observed results.

Recently, fluorescent Silicon (Si) Quantum Dots (QDs) have attracted much interest due to their high quantum yield, use of non-toxic and environmentally-benign chemicals, and water-solubility. However, more research is necessary to understand the energy level characteristics and single molecule behavior to enable their development for imaging applications. Therefore, single molecule time-resolved fluorescence spectroscopy of fluorescent Si QDs (cyan, green, and yellow) is needed. A rigorous analysis of time-resolved photon correlation spectroscopy and fluorescence lifetime data on single Si QDs at room temperature is presented.

We report on the use of time-resolved and polarised evanescent wave-induced fluorescence anisotropy measurements to probe molecular photophysics, motion and energy migration of fluorescent species in close proximity to a silica/film interface. In particular we show that the fluorescence decay and anisotropy of common fluorophores varies as a function of the plane of the fluorophore with respect to the interface, the distance from the interface, and as a function of position (using polarised EW imaging). We have applied time-resolved and polarised EW-induced fluorescence microspectroscopic measurements to dyes, thin polymer nanoparticle films and cells on silica surfaces, probing the variation in the photophysical dynamics within the films.

Resonance Raman Spectroscopy (RRS) is employed in this study to examine the influence of molecular weight on the optical response of a diketopyrrolopyrrole polymer (DPP-TT-T) in solution. The vibronic structure observed for the ground state absorption of this polymer is found to vary with molecular weight and solvent. Resonance Raman Intensity Analysis (RRIA) revealed that the absorption spectra can be described by at least two dipole-allowed transitions and the vibronic structure variation is due to differing contributions from linear and curved segments of the polymer.

Particle-size or ‘quantum-confinement’ effects have been used for decades to tune semiconductor opto-electronic properties. More recently, particle size control as the primary means for properties control has been succeeded by nanoscale hetero-structuring. In this case, the nanosized particle is modified to include internal, nanoscale interfaces, generally defined by compositional variations that induce additional changes to semiconductor properties. These changes can entail enhancements to the size-induced properties as well as unexpected or ‘emergent’ behaviors. Common structural motifs include enveloping a spherical semiconductor nanocrystal, i.e., a quantum dot, within a shell of a different composition. In this talk, I will discuss how solution-phase synthesis can be used to create these structures with precisely ‘engineered’ complexity. Most notably, I will review our experiences with so-called ‘giant’ quantum dots that, due to their internal nanoscale structure, exhibit a range of novel behaviors, including being non-blinking and non-photobleaching (Chen et al. J. Am. Chem. Soc. 2008, 130, 5026; Ghosh et al. J. Am. Chem. Soc. 2012, 134, 9634; Dennis et al. Nano Lett. 2012 12, 5545; Acharya et al. J. Am. Chem. Soc. 2015, 137, 3755), and remarkably efficient emitters of ‘multi-excitons’ due to extreme suppression of Auger recombination (Mangum et al. Nanoscale 2014, 6, 3712; Gao et al. Adv. Optical Mater. 2015, 3, 39). I will discuss recent work extending non-blinking behavior to the blue/green and “dual-color” emission, and show how correlated optical/structural characterization can reveal new information regarding structure-property relations to guide new nanomaterials development (Orfield et al. ACS Nano, Article ASAP).

Upconversion nanoparticles have been demonstrating great potentials in deep-tissue imaging with less or without photon-damage because they can convert the excitation light in the near-infrared wavelength region (700 nm < λex< 1200 nm) to the emission light in the visible region (400 nm < λem< 700 nm). This study presents a facile method for the synthesis of amine functionalized hydrophilic NaGdF4: Yb3+, Er3+ upconverting nanostructures. The prepared nanoparticles were subjected to various characterizations including transmission electron microscopy (TEM) and high-resolution TEM, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD). The fluorescence and magnetic properties of the produced upconversion nanoparticles were further investigated by a fluorometer and a vibrating sample magnetometer (VSM) analysis. It is found that the ratio of the emission intensity of the synthesized upconverting nanoparticles in aqueous at 540 nm to their emission intensity at 660 nm (I540/I660) is about 8.3 times of the ratio (I540/I660) of the upconverting nanoparticle in the format of powder. The difference between the samples in aqueous media and in the format of powder may be caused by the water molecules absorbed onto the surface ligands of the nanoparticles, which is likely to decrease upconversion fluorescence by nonradiative decay. This method provided a facile synthesis route to the preparation of amine functionalized hydrophilic NaGdF4: Yb3+, Er3+ upconverting nanoparticle.

This talk will discuss surface gating of the carrier density in colloidal quantum dots and the applications for infrared detection and emission. Doping of carriers in colloidal quantum dots (CQD) is important for a number of applications. While doping in bulk semiconductors is achieved with heterovalent impurities, CQDs can be doped by charge transfer from outside using reducing reagents or electrochemistry. In addition, the positions of the energy levels inside the particle and the outside Fermi level depend on the polarity of the interface, such that the surface composition can affect doping. This is the concept of surface gating. A striking demonstration is the filling and emptying of the quantum states of HgSe and HgS quantum dots by enrichment of the surface in metal or sulfur atoms. These are the first CQDs to exhibit carrier doping in ambient conditions. CQDs for infrared photodetection or emission is now extended to wider gap systems by using the intraband transitions of the doped quantum dots, first investigated nearly two decades ago.

Multiple exciton generation (MEG) - a process in which multiple charge-carrier pairs are generated from a single optical excitation – is a promising way to improve the photocurrent in photovoltaic devices and offers the potential of breaking the Shockley-Queisser limit. It remains, however, challenging to harvest charge-carrier pairs generated by MEG in working solar cells. Initial yields of additional carrier pairs may be reduced due to ultra-fast intraband relaxation processes, which compete with MEG at early times. Quantum dots of materials, which display reduced carrier cooling rates (e.g. PbTe)[1] or one-dimensional nanostructures (e.g. nano rods)[2] which accelerate the carrier multiplication process are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we show that both theorised strategies can lead to solar cells, which produce extractable charge carrier pairs with an external quantum efficiency above 120%, and we estimate an internal quantum efficiency exceeding 150%. Resolving the charge carrier kinetics on the ultra-fast timescale with pump-probe transient absorption and pump-push-photocurrent measurements, we identify a delayed cooling effect above the experimentally- determined threshold energy for MEG[1].

Hamaker-Lifshitz constants are used to calculate van der Waals interaction forces between small particles in solution. Typically, these constants are size-independent and material specific. According to the Lifshitz theory, the Hamaker-Lifshitz constants can be calculated by taking integrals that include the dielectric permittivity, as a function of frequency, of the interacting particles and the medium around particles. The dielectric permittivity of interacting metal nanoparticles can be calculated using the free-electron Drude model for metals. For bulk metals, the Drude model does is size independent. However, the conducting electrons in small metal nanoparticles exhibit surface scattering, which changes the complex dielectric permittivity function. Additionally, the Drude model can be modified to include temperature dependence. That is, an increase in temperature leads to thermal volume expansion and increased phonon population, which affect the scattering rate of the electrons and the plasma frequency. Both of these terms contribute significantly to the Drude model for the dielectric permittivity of the particles. In this work, we show theoretically that scattering of the free conducting electrons inside noble metal nanoparticles with the size of 1 – 50 nm leads to size-dependent dielectric permittivity and Hamaker-Lifshitz constants. In addition, we calculate numerically the Hamaker-Lifshitz constants for a variety of temperatures. The results of the study might be of interest for understanding colloidal stability of metal nanoparticles.

High Q-factors and small mode volumes have made toroidal optical microresonators exquisite sensors to small shifts in the effective refractive index of the WGM modes. Eliminating contaminants and improving quality factors is key for many different sensing techniques, and is particularly important for photothermal imaging as contaminants add photothermal background obscuring objects of interest. Several different cleaning procedures including wet- and dry-chemical procedures are tested for their effect on Q-factors and photothermal background. RCA cleaning was shown to be successful in contrast to previously described acid cleaning procedures, most likely due to the different surface reactivity of the acid reagents used. UV-ozone cleaning was shown to be vastly superior to O₂ plasma cleaning procedures, significantly reducing the photothermal background of the resonator.

There is a growing demand to improve the operational lifetime of electroluminescent (EL) devices utilizing conjugated polymers which are often deposited over metal electrodes. Photo-degradation of the emissive organic layer is one factor that decreases the overall efficiency and longevity of these devices. Therefore, it is important to investigate the underlying photochemistry at metal-polymer interface. Here, effects of metal films on the emission properties of organic polymers are studied using total internal reflection fluorescence (TIRF) microscopy. It is observed that poly(phenylene vinylene) (MEHPPV) exhibits a remarkable increase in photo-stability when deposited on gold films relative to that on glass even in the presence of molecular oxygen and under continuous laser illumination. It is proposed that this interesting property is due to the surface plasmon of Au films.

Compact computational structure-function relations are needed to examine energy transfer between confined fields and carrier dynamics at heterostructure interfaces. This work used discrete dipole approximations to analyze quasiparticle excitation and dephasing at interfaces between metals and van der Waals materials. Simulations were compared with scanning transmission electron microscopy (STEM) for energy electron loss spectroscopy (EELS) at sub-nanometer resolution and femtosecond timescale. Artifacts like direct electron-hole pair generation were avoided. Comparing simulation with experiment distinguished quasiparticle energy transfer to hot carriers at the interface, and supported development of structure-function relations between interface morphology and emergent discrete and hybrid modes.

Current matching limits the commercialization of tandem solar cells due to their instability over spectral changes, leading to the need of using solar concentrators and trackers to keep the spectrum stable. We demonstrate that voltage-matched systems show far higher performance over spectral changes; caused by clouds, dust and other variations in atmospheric conditions. Singlet fission is a process in organic semiconductors which has shown very efficient, 200%, down-conversion yield and the generated excitations are long-lived, ideal for solar cells. As a result, the number of publications has grown exponentially in the past 5 years. Yet, so far no one has achieved to combine singlet fission with most low bandgap semiconductors, including crystalline silicon, the dominating solar cell material with a 90% share of the PV Market. Here we show that singlet fission can facilitate the fabrication of voltage-matched systems, opening a simple design route for the effective implementation of down-conversion in commercially available photovoltaic technologies, with no modification of the electronic circuitry of such. The implemention of singlet fission is achieved simply by decoupling the fabrication of the individual subcells. For this demonstration we used an ITO/PEDOT/P3HT/Pentacene/C60/Ag wide-bandgap subcell, and a commercial silicon solar cell as the low-bandgap component. We show that the combination of the two leads to the first tandem silicon solar cell which exceeds 100% external quantum efficiency.

Research on carbon-based photocatalytic nanomaterials has been a field in continuous expansion in the last years. Graphene (or its derivatives) is currently one of the most studied materials due to its high surface area, photodegradation resistance, optical transparency and high charge mobility values. All of these excellent properties are highlighted for applications in various research areas. The incorporation of small amounts of reduced graphene oxide (RGO) sheets in semiconductors matrices is also a strategy widely used to improve the physicochemical properties, which cannot normally be achieved using conventional composites or pristine semiconductors. Most studies suggest that these twodimensional (2D) materials can facilitate electron injection and assist the electron transport in semiconductors. In this context, this manuscript will present examples of graphene-based semiconductor nanocomposites obtained by our research group and their application in the photodegradation of methylene blue (MB), photocatalytic conversion of CO₂ to hydrocarbon fuels and photocatalytic water splitting reaction. Our results show the positive effect of coupling the RGO sheets with semiconductors for photocatalysis.

In the last few years, tremendous research interest has been focused on two-dimensional transition metal dichalcogenides, following progress in processing of layers with atomic-size thickness. Among them, molybdenum disulfide (MoS₂) nanoflakes have shown unique optical and electrical properties. They are excellent absorbers, despite being ultrathin, with high exciton-binding energies that make excitonic transitions evident even at room temperature. Bulk and few-layer MoS₂ are indirect band gap semiconductors, while its monolayer is a direct band gap semiconductor. Also, bound trions have been reported in monolayer MoS₂, due to strong interactions between excitons and charges and spatial confinement of the photoexcited species.

Ultrafast spectroscopy has brought important clarification to the aforementioned properties in MoS₂ nanoflakes and has unraveled their carrier dynamics and transport properties. However, the strong dependence of the fundamental properties on the nanoflake size and preparation process clearly shows that additional research is needed to understand the rich and unusual photophysics in this system. Therefore, we have used time-resolved absorption and THz transmission spectroscopy to shed light on the photophysical properties of solution-processed, few-layer MoS₂ nanoflakes with subpicosecond temporal resolution. Using different excitation photon energies and fluences, we have resolved the carrier and exciton relaxation, the recombination processes and the corresponding time scales. Also, we have used the spectrum of the complex photoconductivity in the THz region to study the carrier transport properties in the nanoflakes as a function of number of layers.

In this paper two extremely narrow band-gap polymers, based on naturally occurring indigo with high thin film crystallinity, have been examined using transient absorption spectroscopy. This was done in order to assess their charge photogeneration and recombination characteristics in blends with PC₇₁BM. Two charge photogeneration mechanisms are found to be operating, depending on which component of the blend is photoexcited. Despite virtually isoenergetic LUMO levels, photoexcitation of the polymer causes standard electron transfer, albeit with a relatively low efficiency of 17 %. Photoexcitation of the fullerene, however, produces an exceptionally slow nanosecond timescale hole transfer.

Single-walled carbon nanotubes (SWCNT) are a promising material for future optoelectronic applications, including flexible electrodes and field-effect transistors. Molecular doping of carbon nanotube surface can be an effective way to control the electronic structure and charge dynamics of these material systems. Herein, two organic semiconductors with different energy level alignment in respect to SWCNT are used to dope the channel of the SWCNT-based transistor. The effects of doping on the device performance are studied with a set of optoelectronic measurements. For the studied system, we observed an opposite change in photo-resistance, depending on the type (electron donor vs electron acceptor) of the dopants. We attribute this effect to interplay between two effects: (i) the change in the carrier concentration and (ii) the formation of trapping states at the SWCNT surface. We also observed a modest ~4 pA photocurrent generation in the doped systems, which indicates that the studied system could be used as a platform for multi-pulse optoelectronic experiments with photocurrent detection.

We combine low frequency Raman scattering measurements with first-principles molecular dynamics (MD) to study the nature of dynamic disorder in hybrid lead-halide perovskite crystals. We conduct a comparative study between a hybrid (CH3NH3PbBr3) and an all-inorganic lead-halide perovskite (CsPbBr3). Both are of the general ABX3 perovskite formula, and have a similar band gap and structural phase sequence, orthorhombic at low temperature, changing first to tetragonal and then to cubic symmetry as temperature increases. In the high temperature phases, we find that both compounds show a pronounced Raman quasi-elastic central peak, indicating that both are dynamically disordered.

Organic-inorganic perovskites such as CH3NH3PbI3 are highly promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21% and promising applications in light-emitting diodes, lasers and photodetectors also emerging. A key enabling property of the perovskites is their high photoluminescence quantum efficiency, suggesting that these materials could in principle approach the thermodynamic device efficiency limits in which all recombination is radiative. However, non-radiative recombination sites are present which vary heterogeneously from grain to grain and limit device performance. Here, I will present results where we probe the local photophysics of neat CH3NH3PbI3 perovskite films using confocal photoluminescence (PL) measurements and correlate the observations with the local chemistry of the grains using energy-dispersive X-ray spectroscopy (EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). We investigate the connection between grains that are bright or dark in emission and the local Pb:I ratios at the surface and through the grains. We also examine how the photophysics, local chemistry and non-radiative decay pathways change slowly over time under illumination. Our results reveal a “photo-induced cleaning” arising from a redistribution of iodide content in the films, giving strong evidence for photo-induced ion migration. These slow transient effects appear to be related to anomalous hysteresis phenomena observed in full solar cells. I will discuss how immobilizing ions, reducing trap densities and achieving homogenous stoichiometries could suppress hysteresis effects and lead to devices approaching the efficiency limits.

Hybrid halide perovskites are at the frontier of optoelectronic research due to their excellent semiconductor properties and solution processability. For this reason, much attention has recently been focused on understanding photoexcited charge-carrier generation and recombination in these materials. Conversely, very few studies have so far been devoted to understanding carrier-carrier and carrier-phonon scattering mechanisms in these materials. This is surprising given that carrier scattering mechanisms fundamentally limit charge-carrier motilities and therefore the performance of photovoltaic devices. We apply linear polarization selective transient absorption measurements to polycrystalline CH3NH3PbBr3 hybrid halide perovskite films as an effective way of studying the scattering processes in these materials. Comparison of the photo induced bleach signals obtained when the linear polarizations of the pump and probe are aligned either parallel or perpendicular to one another, reveal a significant difference in spectral intensity and shape within the first few hundred femtoseconds after photoexcitation.

There is mounting evidence that long-range charge separation determines the efficiency of organic photovoltaic cells, yet different mechanisms remain under debate. One class of proposed mechanism is ultrafast coherent long-range charge separation, and another is a slower process whereby charges incoherently hop apart with a transiently enhanced mobility due to morphology and disorder. Here, we use transient absorption spectroscopy to probe incoherent charge separation dynamics in two different ways. First, we use a family of polymers whose backbone structures allows us to compare 2- phase donor-acceptor morphologies with 3-phase morphologies that feature an intermixed region. In the 3-phase system, we see pronounced spectral signatures associated with charges (holes) occupying the disordered intermixed region, and we track separation via biased charge diffusion to more ordered neat regions on the timescale of hundreds of picoseconds. Secondly, by resolving bimolecular charge recombination at high excitation density, we show that charge mobilities must be substantially enhanced on early timescales, which may be sufficient for separation to occur. Together, these measurements provide support for models of incoherent and relatively slow charge separation.

Charge transfer (CT) states play evidently an important role at the interface of organic heterostructures but their identification and characterization is often experimentally less obvious and challenging. We studied two exemplary material systems which both represented a benchmark within the research of organic photovoltaics at their time: the homopolymer P3HT blended with PC61BM and the copolymer PTB7 blended with PC71BM. In both heterostructures, we could identify a distinct CT state emission by the use of NIR time-resolved photoluminescence (PL) [1], [2]. The selectivity of this technique enables us to clearly probe the energetics and dynamics of weak emitting interfacial states and therefore to prove differences in the CT state characteristics between the two systems. We went beyond this previous work and investigated the time and temperature dependent emission anisotropy as well as the electric field dependence of the time-resolved PL for both blends and the pristine polymers, respectively. In both cases the CT state emission clearly deviates from the one of the primarily excited singlet excitons: the emission anisotropy reveals an additional relaxation pathway for the exciton which is connected with a change of the transition dipole moment of the emission, and under applied bias different quenching thresholds can give access to varying binding energies of the emissive excitons involved. Finally, we think that our findings demonstrate how interfacial CT state emission can be clearly identified as such and how it can be unambiguously distinguished from singlet exciton emission.

With the rising popularity of organic light-emitting diodes (OLEDs) in display applications, demand for more efficient blue emitters has increased. We have recently synthesized a novel blue-emitting, donor-acceptor system employing carbazole as the donor and a benzothiazole derivative as the acceptor, BTZ-CBZ. We find that the solution-phase emission of BTZ-CBZ is highly dependent on solvent polarity, both in lineshape and emission maximum, showing a Stokes shift of 50 nm in methylcyclohexane and 150 nm in acetonitrile. This is expected behavior for donor-acceptor compounds due to the presence of a charge-transfer excited state. However, the solid state properties are more important for OLED devices. Using time-dependent density functional theory calculations employing the linear-response (LR) and state-specific (SS) polarizable continuum model (PCM), we explore the effects of solvent reorganization on the emission properties of BTZ-CBZ. SS-PCM reproduces the solvatochromism behavior of BTZ-CBZ in solution, but LR-PCM shows effectively no shift with solvent polarity. We surmise that this is because solvent reorganization is necessary for the solvatochromic effect to occur. The effect of rigid matrices on the emission of BTZ-CBZ has direct implications on its viability as a blue emitter in solid-state OLEDs and which molecular environments will be ideal for devices.